Amperometric gas sensors typically include an electrolytic cell having a pair of electrodes which are immersed in an electrolyte solution and which are exposed to the gas to be measured through a gas permeable membrane. Because of the thinness of the membrane, the cell is sensitive to differences in the pressure between the inside and outside of the cell. As a result, the need for providing a pressure compensation mechanism has long been recognized. One group of approaches to solving this problem is described in U.S. Pat. No. 4,126,531, issued on Nov. 21, 1978, entitled "Sensor With Annular Pressure Compensating Means" and in U.S. Pat. No. 3,577,332, issued on May 4, 1971, entitled "Polarographic Cell." In the former patent, a pressure compensating membrane serves as an elastic compensating cover for a plurality of pressure compensating openings. In the latter patent a movable diaphragm transmits external pressure changes to the interior of the cell by flexing in the manner of a barometer and thereby increasing or decreasing the pressure within the cell. While such approaches may work well in the presence of relatively small changes in pressure, they cannot deal with large pressure changes such as, for example, changes which amount to a substantial percentage of atmospheric pressure.
Other approaches shown, for example, in U.S. Pat. No. 3,510,421, issued on May 5, 1970, entitled "Polarographic Cell" and U.S. Pat. No. 3,929,603, issued on Dec. 30, 1975, entitled "Electrolytic Sensor with Pressure Compensating Means," involve the use of internal or external pressure compensating bladders having substantially fixed volumes. While such structures provide a degree of compensation for changes in the pressure of gases exterior to the cell, they are poorly equipped to deal with changes in pressure resulting from changes in the interior of the cell, such as those that are incident to the thermal expansion and contraction of the cell and its contents.
Still another approach to the solution of the pressure compensation prdblem is shown in U.S. Pat. No. 4,178,223, issued on Dec. 11, 1979, entitled "Electrical Oxygen Probe." In this patent the interior of the cell is simply left open to the atmosphere. While such an approach no doubt prevents the build up of pressure differences, it has the disadvantage that, being unsealed, contaminants can enter through the vents to pollute the electrolyte solution. In addition, the electrolyte can change in concentration as a result of evaporation and thereby produce inaccurate readings.